FIG. 6 is a cross sectional view showing a conventional speaker apparatus. As shown in FIG. 6, the conventional io speaker apparatus includes a ring-shaped magnet 8, a ring-shaped magnetic pole plate 7, a pore yoke 4 having a center pore 4'. With an effect of the ring-shaped magnet 8, a magnetic gap containing a magnetic field is formed between the outer wall of center pore 4' and the inner wall of the ring-shaped magnetic pole plate 7. Disposed in the magnetic gap is a voice coil 6 wound around a bobbin 2. When electric current is flowing through the voice coil 6, the bobbin 2 will be driven (in a vertical direction in the drawing) due to an action of electromagnetic effect in accordance with Fleming's left-hand rule. Thus, a vibrating diaphragm 1 which is attached on a frame 10 through an annular edge 9 and a damper 3, will be vibrated so as to produce a sound corresponding to the current flowing through the voice coil 6. Further, a protection cap 11 is provided on the center of the diaphragm 1 to prevent outside dust from invading into the magnetic pole plate 7.
Referring again to FIG. 6, the damper 3 is a corrugation damper formed by impregnating a woven fabric with a thermosetting resin and then heat-pressing the resin-impregnated woven fabric, or a butterfly-shaped damper formed by properly punching a piece of phenol resin sheet or a metal sheet.
However, a corrugation damper has a problem of aeolotropy. Namely, with a corrugation damper, the magnitude of a holding force will be different from one direction to another. In particular, when there is a large vibration, a voice coil held by the corrugation damper will cause an undesired rolling movement, undesirably contacting with other elements in the speaker apparatus. As a result, it will be difficult to correctly produce a reproduced sound.
On the other hand, although a butterfly-shaped damper made of a phenol resin or metal sheet does not have a problem of aeolotropy, because a butterfly-shaped damper is formed by punching a piece of phenol resin sheet or a metal sheet, there is a wast of useful material (phenol resin or metal sheet), hence the production cost is high.
Recently, it has been required that a loudspeaker should be small in size but capable of reproducing a louder and low voice. In order to obtain a louder and low voice in a small size loudspeaker, it is necessary that the vibration amplitude of a vibrating diaphragm be large. However, with a damper made of a phenol resin or metal sheet, when there is a large vibration amplitude, such a damper is prone to be damaged because there is not a sufficient bending strength or because there is a repeated fatigue. Moreover, since a damper is usually positioned adjacent the voice coil which generates heat, it is also required that a heat resistant material be used to make a damper.
In addition, a damper such as indicated by the reference numeral 3 has a center hole through which a voice coil 6 wound around a bobbin 2 may pass. When assembling a loudspeaker apparatus, after a voice coil 6 has been passed through the center hole, the bobbin 2 is bonded to the damper 3 along the inner circumferential portion thereof. If, in order for the voice coil 6 to easily pass through the center hole, the center hole of the damper 3 is required to have a large diameter, resulting in a problem that an adhesive agent for bonding the bobbin 2 to the damper 3 will flow through an undesirably formed annular gap between the bobbin 2 and the inner wall of center hole, hence making it difficult to form an exact bonding between the bobbin 2 and the damper 3, and thus rendering it difficult for the bobbin 2 to be located at a desired correct position with respect to the damper 3.
On the other hand, if an annular gap between the bobbin 2 and the damper 3 is to be made as small as possible in order to achieve an exact bonding therebetween, it will be difficult for the voice coil 6 to pass through the center hole of the damper 3, or otherwise requiring an extremely high precision when making the damper 3.